feat: Add error handling and input validation to Rust core (#82)

[ada-cinar]

Summary

Adds proper error handling and input validation to all Rust core functions, replacing panics with Pythonic ValueError exceptions.

Changes

✅ Error Handling:

• Updated all #[pyfunction] signatures to return PyResult<T>
• Added PyValueError exceptions for invalid input
• Replaced panics with graceful error messages

✅ Input Validation:

• Empty string checks in fast_normalize, lookup_lemma, strip_suffixes
• Zero min_root_length validation in strip_suffixes_validated
• Graceful handling in tokenize_with_offsets (returns empty list)

✅ Testing:

• Added 16 comprehensive error handling tests
• Covers edge cases, error messages, boundary conditions
• All error handling tests passing (16/16)

✅ Documentation:

• Updated function docs with error specifications
• Added Python examples for error handling

Benefits

1. Pythonic Errors: Clear ValueError messages instead of opaque panics
2. Better DX: Easier debugging and error tracing
3. Best Practices: Follows PyO3 recommended patterns
4. Type Safety: Proper Result propagation

Testing

pytest tests/test_error_handling.py -v
# ✅ 16 passed in 0.64s

pytest tests/ -v
# ✅ 74 passed, 5 skipped, 1 unrelated failure

Closes

Closes #82

Related

• [Test Coverage] Add Rust Unit Tests for fast_normalize and tokenize_with_offsets #73 (Rust unit tests)
• [Test Coverage] Add Integration Test Suite for Pipeline Error Handling & Edge Cases #70 (Pipeline error handling)